As technological development and demand of mobile instruments have been increased, demand of secondary batteries as energy sources has been increased rapidly. Particularly, many studies about lithium secondary batteries having high energy density and discharge voltage among such secondary batteries have been conducted actively. In addition, lithium secondary batteries have been commercialized and used widely.
The lithium secondary batteries are classified, depending on the composition or form of an electrolyte, into lithium ion batteries, lithium ion polymer batteries, lithium polymer batteries, or the like. The lithium ion batteries have a structure in which a cathode/separator/anode is wetted with a lithium electrolyte solution. The lithium polymer batteries use a solid electrolyte as an electrolyte so that the solid electrolyte may also function as a separator. The lithium ion polymer batteries are in the medium between the lithium ion batteries and the lithium polymer batteries, and have a structure in which a cathode and anode are bound to a separator and a lithium electrolyte is injected thereto.
The lithium secondary batteries may be classified, depending on the shape of a battery casing, into a cylindrical battery, prismatic battery and a pouch-type battery. The cylindrical battery and prismatic battery have a structure including an electrode assembly received in a metallic can, and the pouch-type battery has a structure including an electrode assembly received, for example, in a pouch-type casing made of an aluminum laminate sheet.
One of the main study subjects of such secondary batteries is to improve the safety. For example, when a temperature of secondary battery rises due to abnormal operating states, such as an overcharged state exceeding an acceptable current and voltage, or exposure to high temperature, the electrolyte becomes electrochemically unstable and may be decomposed, resulting in deformation of the outer shape of the battery or an internal short-circuit. In a severe case, ignition or explosion of the battery may occur.
Particularly, the safety issue becomes more important because of a tendency to increase the energy density and discharge voltage in order to satisfy the demand of high-output large-capacity secondary batteries. Therefore, various attempts have been made on the technology of consuming charging energy by installing an electric resistance on a circuit carrying current upon the generation of abnormalities of batteries.
However, there is still a need for a solution of safety issue of a lithium secondary battery at high temperature under an abnormal operating condition.